It's impossible to smile while singing some vowel sounds, meaning soprano singers can't hit the high notes loudly, according to new French and Australian research.

They have particular problems with singing high 'oo' and 'or' sounds, French soprano singer and undergraduate physicist Elodie Joliveau found while working with Australian researchers from the University of New South Wales.

They published their results in the current issue of the journal Nature.

Their research involved eight volunteer soprano singers, four who sung professionally, and four advanced students with an average of nine years' classical training.

The researchers were the first to use equipment that incorporated a sound source as well as a microphone to determine how a singer's vocal tract (the area in the throat where sounds are created) varied with pitch. The equipment was first optimised for use with sopranos by testing it out on Joliveau herself.

The sound source was a flexible pipe taped to the singer's throat that simultaneously produced several hundred different sound frequencies. A microphone attached to the singer's lower lip picked up the frequencies that made the singer's vocal tract resonate. The microphone also recorded the singer going through a series of scales with different vowel sounds, such as 'ah', 'or', and 'oo'.

The researchers found that vocal tract resonance and pitch didn't match for rounded-mouthed vowel sounds such as 'or' and 'oo' sung at the extreme end of the range (high B and C).

The singers couldn't open their mouths wide enough to tune to these pitches while still keeping rounded lips. Because of this, rounded vowel sounds at higher pitches were softer and more difficult to differentiate than other vowel sounds such as 'ah' and 'aa'.

Dropping the jaw (or otherwise opening the mouth such as smiling) allows sopranos to increase the frequency at which the vocal tract resonates, so the mouth opens wider as they ascend in pitch.

Wolfe said that in normal speech, or for singers with lower ranges, the resonant frequency of the vocal tract determines the vowel sound while the vocal chords determines the pitch. Varying the two independently allows us to sing different vowel sounds at different pitches.

But sopranos, who sing at very high frequencies, with a vocal range of 300-1000 Hertz, have a volume problem, said Wolfe. If they sing a vowel where their vocal tract resonance coincides with the pitch the note is very loud, and if the resonance doesn't coincide with the pitch the note is very soft.

"Some notes would be loud and some notes soft, which would be unattractive," said Wolfe.

"Sopranos learn to tune their vocal tracts but no-one has been able to measure their resonances while they are singing. That's where we come in because we can do just that."

Sound files showing the effect, as well the original article and a non-technical description, are available on the Music Acoustics web site at University of New South Wales.